Alloy and method



Dec. 19, 1961 Filed June 18, 1958 W. M. FRASER ALLOY AND METHOD FIG. I.

COMPACT MIXTURE TO FORM SELF-SUSTAINING GREEN INGOT SINTER GREEN INGOT lN NON-OXID IZING ATMOSPHERE MECHANICALLY REDUCE SINTERED INGOT INTO ELONGATED FORM 2 Sheets-Sheet 1 IN V EN TOR.

WILL/HM M. FE/ISEZ.

MVENTd/F Dec. 19, 1961 w. M. FRASER 3,01

ALLOY AND METHOD Filed June 18, 1958 2 Sheets-Sheet 2 FIG.2.

FIG. 3.

1M 'EN TOR. W/LL/Hf'7 M. FRASER BY 9 PM 3,013,329 ALLOY AND METHOD William M. Fraser, Cedar Grove, Ni, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed June 18, 1958, Ser. No. 742,948 4 Claims. (Cl. 29182.5)

This invention relates to alloys and, more particularly, to a molybdenum-silica alloy and to a method for making same.

Molybdenum has, very good high-temperature characteristics in that it displays reasonable ductility, hardness and tensile strength at relatively high temperatures. This makes its use particularly desirable in such applications as jet engine parts, rocket parts, etc. Above a temperature of about 1100" C. the usual molybdenum metal recrystallizes and on recrystallization displays a considerably decreased tensile strength and hardness. In explanation of the term recrystallization, the usual molybdenum is formed by powder-metallurgy practices or techniques wherein molybdenum oxide is first reduced to molybdenum powder. The powder is then compacted to form a self-sustaining green ingot and the green ingot is sintered in a hydrogen furnace to form a molybdenum ingot which can be mechanically elongated in configuration without fracturing, or otherwise expressed, the ingot can be reduced in cross-sectional area without fracturing. This sintered ingot is then either rolled or swaged and drawn according to whether sheet, rod or wire is being fabricated and the resulting sheet, rod or wire as the case may be will display what is known in the art as a worked structure, resulting from the mechanical working. Upon heating to temperatures greater than about 1100 C., however, the worked structure disappears through the formation of crystals of the metal and when this occurs, the desirable properties of the molybdenum aredeleteriously affected. For some special applications it is desirable to utilize molybdenum parts or components which have a considerably higher recrystallization temperature in order that such components will display the hardness, etc., as are obtained with unrecrystallized molybdenum, even when the temperatures exceed the usual recrystallization temperature of molybdenum.

It is the general object of this invention to avoid and overcome the foregoing and other ditiiculties of and objections to the prior-art practices by the provision of a molybdenum-silica alloy which has a much higher recrystallization temperature. g

It is another object to provide a method for fabricating a molybdenum-silica alloy which has a much higher recrystallization temperature.

It is a further object to provide permissible and preferred ranges for components comprising such a molybdenum-silica alloy as 'well' as permissible and preferred methods for fabricating this alloy.

The foregoing objects of the invention, and other ob jects which will become apparent as the description proceeds, are achieved by providing a molybdenum-silica alloy containing from about 98.9% to about 99.9% by weight molybdenum and from about 1.1% to about 0.1% by Weight silica. There is also provided a method for making such an alloy. This alloy can be maintained at very high temperatures such as 1630" C. without recrystallizing.

For a better understanding of the invention, reference should be had to the accompanying drawings wherein:

FIG. 1 is a flow chart showing the method of preparing the alloy;

FIG. 2 is a photomicrograph taken with a magnification of 500 showing an etched cross section of wire fabricated from the usual prior-art molybdenum after such ice wire has been maintained at a temperature of 1630 C. for 1 hour;

FIG. 3 is a photomicrograph taken with a magnification of 500 showing an etched cross section of wire fabricated from the usual prior-art molybdenum after such wire has been maintained at a temperature of 1630 C.

for two hours;

FIG. 4 is a photomicrograph'taken with a magnification of 500 showing an etched cross section of wire fabricated of the molybdenumsilica alloy of this invention after such wire has been maintained at a temperature of 1630" C. for two hours.

In preparing the molybdenum-silica alloyin accordance with the flow chart as shown in FIG. 1, there is first formed an admixture of finely-divided molybdenum and silicon compound wherein the molybdenum content is from about 98.9% to about 99.9% by weight 'andthe content of silicon expressed as silicon dioxide is from about 1.1% to about 0.1% by weight. Desirably, the molybdenum content in the admixture is from about 99.6% to about 99.8% by weight and the content of silicon expressed as silicon dioxide is from about 0.4% to about 0.2% by weight. Such an admixture is readily formed by adding a silicon salt such as potassium or sodium metasilicate to molybdenum dioxide with the content of silicon expressed as silica so selected as to fall 7 within the foregoing ranges. Other silicon salts can 'be utilized or the silicon can be added as orthosilicic acid, for example, to the molybdenum dioxide. As a specific example, potassium metasilicate is added to finely-divided molybdenum dioxide as a 0.12 normal water solution in amount of 1 cc. per 4.3 grams molybdenum dioxide powder to form a slurry. Thereafter the slurry is evaporated to dryness and desirably is screened to remove overly-large particles, such as by screening through a 40-mesh screen for example. The dry admixture is then reduced in a hydrogen atmosphere at a temperature of 1100 C. for six hours for example to form a thorough admixture of finely-divided molybdenum and silicon compound. The reducedadmixture will normally contain some residual potassium, although an appreciable portion of the potassium silicate will be converted to silicon dioxide by the reduction step. i

As a possible alternative embodiment, orthosilicic acid can be added directly to finely-divided molybdenum powder to the prescribed amount of silicon expressed as silicon dioxide while desirably maintaining the molybdenum powder in a non-oxidizing atmosphere. Thereafter on drying the molybdenum powder-silicic acid admixture, a thorough admixture of finely-divided molybdenum and silica will be obtained.

The molybdenum powder-silicon compound admixture is then compacted to form a self-sustaining green ingot. The ingot dimensions are not critical and can vary over a wide range. As an example, for processing molybdenum rod or wire, a 2500 gram ingot is prepared with a compacting pressure of 18 tons per square inch. As a further example, for processing sheet molybdenum, 3500 grams of the admixture is compacted with a pressure of 17 tons per square inch to form a self-sustaining green ingot. The green ingots are then sintered in a nonoxidizing atmosphere at a temperature of from 1640 C. to 1680 C. for from 2 /2 to 3 /2 hours. The preferred sintering procedure is the usual sintering as effected in a hydrogen atmosphere although other non-oxidizing atmospheres such as the noble gases can be used. Best results have been obtained when the hydrogen atmosphere is wet, that is, when the hydrogen atmosphere is saturated at a temperature of 25 C. The moisture in the hydrogen can be varied considerably and dry hydrogen can be used if desired. Thesintering temperatures and sintering times are not critical and can be varied con-.

Patented Dec. 19, 1961 siderably as is well known. A proper sintering schedule is primarily dependent on a time-temperature relationship and is also somewhat dependent on ingot size and the later working schedules. The purpose of the ingot sintering step is to produce a sintered ingot which can be mechanically elongated in configuration without fracturing and the foregoing sintcrhig schedule for the ingots as specified will produce such sintered ingots.

It should be noted that tungsten as prepared by powder metallurgy practices has been provided with so-called doping constituents which include silicon compounds. The siuterin temperatures utilized with tungsten, however, are normally in excess of 2300 C., at which temperature most of the constituents of the silicon compounds are volatilized. Thus where tungsten has been provided with so-called silica-doping constituents, the most of these doping constituents are lost during sintering and only very slight residual traces remain. In sintering the present molybdenurn-silica alloy, the sintering temperatures utilized are considerably below the temperature at which the silicon will volatilize from the ingot with the result that substantially all the silicon compound present in the ingot converts to silica, which remains in the sintered ingot. Any cation constituent in the silicon compound, such as sodium or potassium, is appreciably volatilized from the ingot during the sintering process and only relatively small amounts of the sodium or potassium, for example, remain in the sintered ingot. These very small residual amounts are not eifective to any appreciable degree in changing the recrystallization temperature of the molybdenum-silica alloy.

As a specific example, for preparing molybdenum rod or wire, the sintered ingot has dimensions of 20 inches by 0.845 inch by 0.905 inch. This sintered ingot is mechanically worked or reduced to an elongated configuration by a series of swaging and drawing operations. While the individual swaging and drawing operations are subject to considerable variation as is well known, the following constitutes a suitable swaging and drawing schedule. This swaging and drawing schedule can be interrupted to achieve any desired diameter of rod or wire.

Hand Swage ingot at a temperature of 1425 C. in a hyrogen atmosphere through three swaging passes to reduce ingot cross-section diameter first to 0.94 inch, then to 0.785 inch and then to 0.640 inch. Remaining swaging and drawing conducted in an air atmosphere.

Swage first to diameter of 0.550 inch and then to 0.447

inch at 1250" C.

Swage to 0.372 inch and then to 0.309 inch at 1200 C.

Swage to 0.256 inch at 1100 C.

Swage to 0.214 inch and then to 0.186 inch at 1000 C.

Draw to 0.004 inch at a temperature of 400 C. through 36 drawing passes, with approximately to reduction in diameter on each pass.

During the foregoing swaging and drawing operations, the material is annealed at 0.309 inch for three minutes at a temperature of 1700 C., at 0.065 inch for two hours at a temperature of 1200 C. and at 0.028 inch for five minutes at 1700" C., all annealing being conducted in a hydrogen atmosphere.

As a specific example for rolling molybdenum sheet from a 3500 gram sintered ingot having dimensions of inches x 1.3 inches x 0.9 inch, the following constitutes a suitable rolling schedule:

Cross roll in a hydrogen atmosphere at a temperature of 1400 C. to a thickness of 0.210 inch.

Longitudinally roll in a hydrogen atmosphere at a temperature of 1100 C. to a thickness of 0.120 inch. All subsequent rolling operations are longitudinal and are conducted in an air atmosphere.

Roll to 0.090 inch at 900 C.

Anneal one hour at 900 C. in a hydrogen atmosphere.

4 Roll to 0.040 inch at 600 C. Anneal one hour at 900 C. in a hydrogen atmosphere. Roll to 0.030 inch at 550 C. Roll to 0.020 inch at 250 C. Anneal one hour at 900 C. in a hydrogen atmosphere.

The foregoing swaging and drawing or rolling schedules essentially involve mechanically working or reducing the sintered ingot into an elongated form. v The resulting formed sheet, rod or wire as the case may be will display what is known as a worked structure. In explanation, an etched cross section of the resulting worked material will display a fibrous grain structure which is greatly elongated -'in the direction of working; This fibrous grain structure imparts to the worked molybdenum a very high tensile strength and hardness and in the vernacular of the art, the molybdenum is known as tough.

In controlled tests conducted with respect to the increased recrystallization temperature for the present molybdenum-silica alloy, fine wire having a diameter of 0.006 inch was utilized, in order to accentuate recrystallization. One lot of this wire was prepared from the present molybdenum-silica alloy. A control lot of similar wire was prepared from the usual prior-art molybdenum. When this fine prior-art molybdenum wire was maintained at a temperature of 1630 C. for about five minutes, recrystallization occurred. In FIG. 2 is shown a photomicrograph of an etched section of such wire after it was maintained at a temperature of 1630 C. for a period of one hour. As illustrated in this photomicrograph, the molybdenuin has recrystallized to form what is known in the art as an equiax crystal structure and the tensile strength and hardness of this molybdenum wire are considerably decreased as compared to the same wire before recrystallization. I

In FIG. 3 is shown a photomicrograph of the same wire as used in taking the photomicrograph shown in FIG. 2, except that the fine wire was maintained at a temperature of 1630 C. for a period of two hours. As shown in this photomicrograph, the crystals have grown further and the equiax crystal structure is still-further accentuated.

In FIG. 4 is shown a photomicrograph of the present molybdenum-silica alloy which has been drawn to wire having a diameter of 0.006 inch, the same as the control wire utilized in taking the photomicrographs in FIGS. 2 and 3. This wire was maintained at a temperature of 1630 C. for two hours, as in the case of the wire as shown in FIG. 3, and after this period of time, the wire formed from the molybdenum-silica alloy still displayed its so-called worked structure, that is, the wire displayed a fibrous grain structure greatly elongated in the direc= tion of swaging and drawing. In further controlled tests, this wire was maintained as long as six hours at 1630 C. with no visible or measurable tendency toward recrystallization.

As based on further controlled tests, the present molybdenum-silica alloy apparently does not recrystallize to any appreciable degree until such temperature is reached that the silica is caused to migrate from the alloy, such temperature being in the order of about 2300 C. Apparently the silica acts as a barrier to the normal grain growth which occurs during recrystallization and this explanation is borne out by the fact that no substantial recrystallization occurs in the present alloy until such temperature is reached thatthe silica is caused to migrate from the alloy. While the foregoing tests were conducted on wire, equivalent increased recrystallization temperature is also obtained with respect to sheet.

The present molybdenum-silica alloy substantially cornprises finely-divided molybdenum and silica within the aforementioned ranges. If the silica is added initially as sodium or potassium metasilicate for example, a relatively small amount of the sodium or potassium may remain in the alloy. Small traces of impurities of other metals such as iron or magnesium for example can be tolerated and are often present. These do not appear to elfect deleteriously the high temperature performance ofthe present molybdenum-silica alloy. Also, while the present alloy substantially comprises molybdenum and silica within the ranges as specified, the presence of some limited amounts of molybdenum-silicon complexes should not be pre cluded. It is further noted that where the silica is present in the alloy in amounts of about 1.3% by weight and greater, the resulting alloy becomes brittle and difiicult to work.

It will be recognized that the objects of the invention have been achieved by providing a molybdenum-silica alloy which has a much higher recrystallization temperature than the usual molybdenum metal. There has also been provided a method for fabricating a molybdenumsilica alloy which has such higher recrystallization temperature and there are further provided permissible and preferred ranges for the components comprising this alloy as well as permissible and preferred methods for fabricating such an alloy.

While best embodiments of the invention have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim:

1. A metallic alloy formed by powder-metallurgy practice wherein alloy constituents are thoroughly admixed, compacted, sintered and mechanicallyelongated in configuration by working, said alloy containing from about 98.9% to about 99.9% by weight of molybdenum and from about 1.1% to about 0.1% by weight of silica, and said alloy when maintained at a temperature of 1650 C. for two hours displaying a fibrous grain structure greatly elongated in the direction of working.

2. A metallic alloy formed by powder-metallurgy practice wherein alloy constituents are thoroughly admixed, compacted, sintered and mechanically elongated in configuration by working, said alloy containing from about 99.6% to about 99.8% by weight of molybdenum and from about 0.4% to about 0.2% by weight of silica, and said alloy when maintained at a temperature of 1650 C. for two hours displaying a fibrous grain structuregreatly elongated in the direction of working.

3. The method of increasing the recrystallization temperature of molybdenum, which method comprises, forming a thorough mixture substantially comprising finelydivided molybdenum and at least one of the group consisting of silicon dioxide and silicon compound at least substantially completely convertible to silicon dioxide and volatile matter when heated with finely divided molybdenum under non-oxidizing conditions to elevated temperatures sufl'icient to sinter molybdenum, with the ratio by Weight of molybdenum to silicon expressed as silicon dioxide being from about 98.9%/1.1% to about 99.9%/

0.1%, compacting said mixture to form a self-sustaining green ingot, sintering said green ingot in a non-oxidiz ing atmosphere at a predetermined temperature and for a predetermined time sufiicient to enable said ingot to be mechanically elongated in configuration without fracturing, and mechanically working said sintered ingot into elongated form.

4. The method of increasing the recrystallization temperature of molybdenum, which method comprises, forming a thorough mixture substantially comprising finely-divided molybdenum and at least one of the group consisting of silicon dioxide and silicon compound at least substantially completely convertible to silicon dioxide and volatile matter when heated with finely divided molybdenum under non-oxidizing conditions to elevated temperatures sufficient to sinter molybdenum, withthe ratio by weight of molybdenum to silicon expressed as silicon dioxide being from about 99.6% 0.4% to about 99.8%/ 0.2%, compacting said mixture to form a self-sustaining green ingot, sintering said green ingot in a non-oxidizing atmosphere at a predetermined temperature and for a predetermined time sufiicient to enable said ingot to be mechanically elongated in configuration'without fracturing, and mechanically working said sintered ingot into elongated form.

References Cited in the file of this patent UNITED STATES PATENTS Weiger May 8, 1954 Streicher July 2, 1956 7 Metals, pages 73-75. 

1. A METALLIC ALLOY FORMED BY POWDER-METALLURGY PRACTICE WHEREIN ALLOY CONSTITUENTS ARE THOROUGHLY ADMIXED, COMPACTED, SINTERED AND MECHANICALLY ELONGATED IN CONFIGURATION BY WORKING, SAID ALLOY CONTAINING FROM ABOUT 98.9% TO ABOUT 99.9% BY WEIGHT OF MOLYBDENUM AND FROM ABOUT 1.1% TO ABOUT 0.1% BY WEIGHT OF SILICA, AND 